The invention relates to a method for dimming a fluorescent lamp arranged in the secondary circuit of a transformer, in which the transformer is driven in a push-pull manner with pulse trains of adjustable duration on the primary side, and also to an arrangement for carrying out the method.
For many applications it is necessary for the brightness of fluorescent lamps to be varied over a large range of adjustment. That is important for example in the context of illuminating display devices in motor vehicles. Such a large range of adjustment is necessary in this case owing to the great fluctuations in the ambient light, in particular between day and night.
Arrangements for operating a fluorescent lamp with controllable luminance are known in which the discharge path of the fluorescent lamp is connected to a controllable AC source via a transformer. The current of the current source is fed to the primary winding of the transformer via two switches operating in a push-pull manner. The switches are in this case driven by a push-pull generator.
Cold-cathode fluorescent lamps have an ignition delay time, that is to say that after the application of an AC voltage to the lamp, a certain period of time lapses before the cold-cathode fluorescent lamp emits light. This ignition delay time is not constant and, moreover, is temperature-dependent. With sine-beat driving of the cold-cathode fluorescent lamp and a lamp that is dimmed to great extent, the fluctuation in the ignition delay time is greater than the period of time in which the lamp emits light. Flickering of the fluorescent lamp occurs as a consequential phenomenon.
In accordance with WO 97/03541, the period of time in which the fluorescent lamp is lit is controlled in order, in this way, to minimize the flickering of the fluorescent lamp.
U.S. Pat. No. 5,311,104 proposes applying an additional ignition pulse, generated by a separate ignition device, to the fluorescent lamp in each case before the beginning of the individual pulse trains.
In accordance with DE 42 22 634 A1, a method for operating a fluorescent lamp is known in which there are generated an AC voltage having a relatively high frequency as operating voltage and a square-wave voltage having a relatively low frequency with an adjustable pulse width for periodically inhibiting and enabling the AC voltage at the fluorescent lamp. In this case, the effect achieved with the aid of a delay device is that the undelayed pulses trigger the ignition pulses, while the delayed pulses lead to the interruption of the electric circuit.